Photon detectors. J. Va vra SLAC

Transcription

1 Photon detectors J. Va vra SLAC

2 Content Comment on timing strategies Vacuum-based detectors: - Hamamatsu MaPMTs - Burle MCP-PMTs with 25 and 10 µm dia. holes Gaseous-based detectors: - Micromegas + MCP Future developments 12/4/05 J.Va'vra, Japan

3 What detector do we want? Present prototype: Future Fast Focusing DIRC: We want to measure x, y and TOP (timeof-propagation) for each photon. We need a single photon timing resolution at a level of ~ ps, to be able to perform the TOP measurement and correct the chromatic error contribution to the Cherenkov angle. We need to operate at 15kG for the Super B-factory, or even at higher field, if the device would find a use at ILC. We want to have a highly pixilated detector. We started with a square pixel size of ~6x6mm. Now we aim for a rectangular size of ~2x8mm. The detector should have a good aging performance. 12/4/05 J.Va'vra, Japan

4 Single photoelectron timing resolution at B = 0 kg

5 High Resolution Timing We have tried these timing techniques: - Leading edge discriminator + single TDC + ADC correction - Constant fraction discriminator (CFD) + single TDC - Two leading edge discriminators with two TDCs per channel Note: There is no evidence that one method is better than the others. We have chosen the CFD method for the Focusing DIRC prototype. But, in retrospect, I think that for alarge scale system, the double-threshold + two TDCs might be a better. Amplifier rise time must be comparable to the photon detector s rise time, and both have to be fast. Need to have expensive tools: - PiLas laser diode with 35ps FWHM timing capability - Fast SiPMT to verify its correct timing operation - 2D-scanning setup to measure a PMT response across its face 12/4/05 J.Va'vra, Japan

6 Speed of the amplifier & detector is essential for good timing From V. Radeka talk at RICH /4/05 J.Va'vra, Japan

7 Examples of two amplifiers Elantek amplifier: - Gain ~130x, MCP-PMT with 25µm holes connected - A ~ 5mV - (ds o /dt) t=0 ~ 0.3V/1ns - t ~ (5x10-3 /0.3)*1ns ~15-20ps Ortec VT-120A amplifier: - Gain ~200x, MCP-PMT with 10µm holes connected - A ~ 5mV - (ds o /dt) t=0 ~ 1.2V/1ns - t ~ (5x10-3 /1.2)*1.0ns ~ 4-5ps Both amplifiers will do the excellent job from noise point of view. However, the Ortec VT-120A amp is much better match for the speed of the MCP-PMT with 10 µm holes. 12/4/05 J.Va'vra, Japan

8 PiLas laser diode and fiber optics Achieved ~ 40-70ps with: - 635, 430 and 407nm wavelengths - 63µm dimeter multi-mode fiber - 5 & 10 m fiber lengths - 1-to-3 fiber splitter - Home-made alignment with the x&y small stage - Mylar attenuators to get single photons - CFD discriminator or TDC/ADC electronics 12/4/05 J.Va'vra, Japan

9 Use a SiPMT detector to verify that the PiLas laser diode SiPMT: CFD analog out, 1ns/div: Use this one in this test Detector: 100 µm dia. GaP SiPMT (APD) operating in a Geiger mode with active quenching. APD developed by Sopko & Prochazka, CVUT Prague. The authors quote this timing resolution: diode ~ (FWHM = 58/2.35) ~ 25 ps for the single photoelectron regime. Therefore, we expect: PiLas ~ sqrt( result2 - APD2 - electronics2 ) ~ sqrt( ) ~23 ps; PiLas data sheet quotes: (35/2.35) ~15ps) - a small inconsistency due to some systematic error ( PiLas power set to ~11% might be too low). Electronics chain in this test: SLAC CFD, 30mV threshold, CFD analog output to the LeCroy 2228ATDC (25ps/count). 12/4/05 J.Va'vra, Japan

10 Hamamatsu H-8500 Flat panel MaPMT Hamamatsu Co. data sheet + SLAC measurements + my interpretation 12/4/05 J.Va'vra, Japan

11 Burle MCP-PMT parameter list Burle Co. data sheet + SLAC measurements + my interpretation!! 12/4/05 J.Va'vra, Japan

12 Timing studies in MaPMT and MCP-PMT Hamamatsu Flat Panel H8500 PMT: Burle MCP-PMT: MaPMT #2 MCP-PMT #3 Double Gaussian fit Burle MCP-PMT #3 has a very long tail due to recoil electrons from the MCP top surface. The tail contains ~20% of all events!!! The MCP-to-cathode distance is 6-7mm. Electronics chain used in this test: Final SLAC amplifier, final SLAC CFD providing the analog output to LeCroy 2228A TDC (25ps/count). Light source: Use the 635nm PiLas laser diode in a single photoelectron mode. 12/4/05 J.Va'vra, Japan

13 Dependence on the MCP PMT design Old design ( ): MCP-to-Cathode distance = 6 mm New design ( ): MCP-to-Cathode distance = 0.75 mm MCP-PMT #3 MCP-PMT #16 Double Gaussian fits. The reduction of the MCP-to-Cathode distance to 0.75mm limits the rate of recoiling photoelectrons from the MCP surface, which reduces the tail in the timing spectrum. These electrons are, however, lost from the detection efficiency, but the spectrum is more Gaussian. Nevertheless, tails would complicate the analysis, and we prefer to cut them. Electronics chain used in this test: Final SLAC amplifier, final SLAC CFD, LeCroy 2228A TDC (25ps/count). Light source: PiLas laser diode in the single photoelectron mode (635nm). 12/4/05 J.Va'vra, Japan

14 Ideal goal: no tails in the distributions MCP-PMT #16 (64 pixels) Double Gaussian fits. No tail in this type of MCP-PMT. Some pixels are better than others. Not clear why. 12/4/05 J.Va'vra, Japan

15 However, the new tube is inefficient around the edges New design ( ): MCP-to-Cathode distance = 0.75 mm The efficiency drops to zero half way through all edge pads. This inefficiency is related to the electrostatic design near the edges. Perhaps, one can have a small light collector around the boundary 12/4/05 J.Va'vra, Japan

16 Compare timing distribution on two different pads with the Phillips 7186 TDC MCP-PMT #16, Pad 14: MCP-PMT #16, Pad 24: Single Gaussian fit to the timing distribution generated in each laser head location. Measure typically = 70-80ps in the central pad region, slightly worse near the boundary. Worse timing resolution around edges is due to the charge sharing, causing lower pulse height, and possibly a cross-talk from hits in neighboring pads. Electronics chain in this test: final SLAC amplifier, final SLAC 32-channel CFD, Phillips 7186 TDC (25ps/count). Detector in this test: MCP-PMT #16 with MCP-to-Cathode distance of 750µm, 8x8 pads, 2.6kV. Light source in this test: PiLas laser diode in the single photoelectron mode (635nm). 12/4/05 J.Va'vra, Japan

17 Single photoelectron timing resolution at B = 15 kg 12/4/05 J.Va'vra, Japan

18 Burle MCP-PMT with 10µm holes 4-pixel MCP-PMT P01 tube for the initial tests. PMT has two MCPs with 10 µm dia. holes Cathode-to-MCP distance ~6mm According to Burle, this particular 10µm MCP should produce a gain of ~10 6 at 2.2kV. Setup had a capability to measure sensitivity to angles in 5 o steps between the magnetic field and axis perpendicular to the face plate. 12/4/05 J.Va'vra, Japan

19 Choice of amplifier and timing results at B = 0 kg 500mV/div, 1ns/div, 2.2kV: Ortec VT-120A amplifier, gain of 200x, (ds o /dt) t=0 ~ 1.2V/1ns Philips CFD discriminator and LeCroy TDC with 25ps/count. Elantek 130x amplifier with 1.5ns risetime gives a smaller pulse height. The detector controls the choice of amplifier: If the amplifier is too slow compared to the detector, one reduces the maximum peak amplitude for a given gain. On the other hand, if the amplifier is much faster than the detector, one increases the noise. 12/4/05 J.Va'vra, Japan

20 Timing results at B = 15 kg 2.7kV Ortec VT-120A amp Initially, there was some confusion what the maximum allowed voltage. Burle initially thought that it is -2.4kV. After I have overvoltaged the tube to -2.7kV to get a decent timing result at 15kG, Burle corrected the max voltage value to kV. I could have gone higher. This means that it is possible to reach a resolution of ~50ps at 15kG. 12/4/05 J.Va'vra, Japan

21 Sensitivity to MCP voltage at B = 15kG Ortec VT-120A amp, -2.65kV, 50mV/div, 1ns/div: The necessary voltage to get a good timing resolution is kV. 12/4/05 J.Va'vra, Japan

22 Sensitivity to angular rotation at B = 15kG Ortec VT-120A amp, -2.65kV, 100mV/div, 1ns/div: The MCP can be tilted by 3-5 o before pulse height is affected. At 10 o, one sees a clear reduction of pulse height, but the tube can still be used. At 15 o and above, the response is killed entirely. 12/4/05 J.Va'vra, Japan

23 Single photoelectron spatial response at B = 0 kg 12/4/05 J.Va'vra, Japan

24 Scanning setup to measure the PMT spatial response x&y stage for the fiber final focus : Stepper motor moves the end of the fiber equipped with a lens, resulting in the spot size of ~150 µm. The linear motor is set typically to: x-step ~ 100µm & y-step ~ 1mm. Light source: - PiLas laser diode operating in single photoelectron mode & 430 nm (on loan) & 407 nm (now). - Fiber is 63µm dia. multi-mode fiber, equipped with lenses at both ends. Analysis: - A hit is accepted into the efficiency definition if it is within a time window, and it is on the same pad as the laser head is pointing to. - To get a relative efficiency we normalize to the 2 inch dia. Photonis XP 2262B PMT ( or the DIRC PMT, ETL 9125FLB17). - DAQ trigger rate: 20kHz.. 12/4/05 J.Va'vra, Japan

25 Resolution of the scanning system Hamamatsu Flat Panel H8500 MaPMT #2: Micro-structure of the dynode electrodes: Resolution: Clearly see the details of the dynode electrode structure. Spatial resolution of the system is less than 100 µm, for a step size of 25µm. Electronics chain used in this test: Final SLAC amplifier, LeCroy 4413 discriminators with 100mV threshold, LeCroy 3377 TDCs with 0.5ns/count. 12/4/05 J.Va'vra, Japan

26 An example of the relative response along a line scan across eight pads Hamamatsu Flat Panel H8500 PMT #2: Burle MCP-PMT #3: The Hamamatsu MaPMT uniformity is ~1:2.5 and the Burle MCP-PMT uniformity is ~1:1.5, in this example. Electronics chain used in this test: Final SLAC amplifier, LeCroy 4413 discriminators with 100mV threshold, LeCroy 3377 TDCs with 0.5ns/count. 12/4/05 J.Va'vra, Japan

27 Burle MCP-PMT #8 relative detection efficiency (Normalized to the Photonis XP 2262B PMT) 635nm: At 635nm, which is close to the end of the Bialkali Q.E. range, the relative efficiency scaling to the Photonis PMT is not very reliable. 430nm: At 430nm, the relative efficiency is 50-60% relative to the Photonis PMT, if we include the late arrivals. This is approximately expected based on the MCP design (to be compared with the geometrical MCP collection efficiency (cathode-to-top MCP) of 60-65%, shown on page 6). Electronics chain used in this test: Final SLAC amplifier, LeCroy 4413 discriminators with 100mV threshold, LeCroy 3377 TDCs with 0.5ns/count Light source: PiLas laser diodes operating in the single photoelectron mode (635nm & 430nm). 12/4/05 J.Va'vra, Japan

28 Relative response across the MCP-PMT face Burle MCP-PMT # nm: 430nm: Burle MCP-PMT # nm: 430nm: Burle MCP-PMT # nm: 430nm: Burle MCP-PMT # nm: 430nm: Typical relative efficiency is 50-60% of the 2 inch dia. Photonis XP 2262B PMT at 430nm. The efficiency drops to 30-50% around the edges at 430nm. 12/4/05 J.Va'vra, Japan

29 Relative response across the MaPMT face Burle MCP-PMT # nm: 430nm: Hamamatsu MaPMT #1 635 nm: 430nm: Hamamatsu MaPMT #2 635 nm: 430nm: Hamamatsu MaPMT #4 635 nm: 430nm: MCP-PMT #16 has large inefficiency around edges (it has the MCP-to-cathode distance of 0.75 mm). Hamamatsu Flat Panel MaPMT relative efficiency is 50-70% of the Photonis XP 2262B PMT at 430nm. The efficiency drops to 30-50% around the edges at 430nm. 12/4/05 J.Va'vra, Japan

30 Aging of MCP-PMT

31 Aging of MCP-PMT Aging due to damage of the photocathode by ion bombardment. Burle claims a ~50% loss after of ~10 C/25cm 2 area of MCP-PMT. Example: DIRC single photon background rate is: ~200 khz per 1 dia PMT at a luminosity of ~10 34 cm -2 sec -1. If I assume that ~1/3 comes from the bar, we run ~6 months/year, then after 10 years, I get about ~10 13 pe - /cm 2. This translates to ~ 1-2 C/25cm 2, if we would have the MCP-PMTs in the present DIRC. The rate is dominated by the LUMI-term, caused by the radiative Bhabhas striking beam components. Nobody knows how to scale things for the Super B factory with a luminosity of > cm -2 sec -1, however, it is clear that one has to pay attention to this problem. 12/4/05 J.Va'vra, Japan

32 Coherent resonance effects? (observed in the prototype) This is what may happen when one tries to be too fast 12/4/05 J.Va'vra, Japan

33 Coherent excitation resonance effects During the run we typically get 3-4 Cherenkov photons, which do not arrive at the same time, so we probably do not suffer from this problem. However, this needs to be fixed. The effect generated by a PiLas producing enough light that multiple pixels fire. At a power of 25% we get a 10% probability to get a hit, which means that something like 6-7 pixels fire per one PiLas trigger. The pulses arrive to the MCP-PMT within < 1 ns, and are capable to excite the standing resonance. 12/4/05 J.Va'vra, Japan

34 Coherent excitation resonance effects The effect does not exist with the Hamamatsu MaPMTs (the same amplifier, the same LV PS, the same grounding). 12/4/05 J.Va'vra, Japan

35 Future developments with the non-gaseous detectors 12/4/05 J.Va'vra, Japan

36 R&D related to Focusing DIRC Develop rectangular pads of 2mm x 8mm, or 3mm x 12mm in size. Suppress the timing tails by reducing the gap between the photocathode and MCP surface. Do more tests with 10µm dia. hole MCP-PMTs in the magnet and estimate better the max possible field. Test the timing with a gaseous MCP + Micromegas photon detector equipped with the Bialkali photocathode. SiPMTs? Rate and aging tests. 12/4/05 J.Va'vra, Japan

37 New 1024-pixel Burle MCP A proposal how to connect pads: Large rectangular pad: 2x8 little ones Small margin around boundary 1024 pixels (32 x 32 pattern) Small pixel size: ~1.4mm x 1.4mm Pitch: 1.6 mm 12/4/05 J.Va'vra, Japan

38 New 256-pixel Hamamatsu MaPMT H-9500 A proposal how to connect pads: Large rectangular pad: 1x4 little ones 256 pixels (16 x 16 pattern). Pixel size: 2.8 mm x 2.8 mm Pitch of 3.04 mm. Very neat connections 12/4/05 J.Va'vra, Japan

39 Open area Burle MCP Small margin around the boundary 10 & 25 µm MCP hole diameter 64 pixel devices Pad size: 6 mm x 6 mm. The MCP-PMT still has 6-7mm cathode-to-mcp distance, thus making a long tail in the timing distribution Can change the resistor chain. Will study if the tail can be supressed by a choice of the MCP operating voltages. Elantek amplifier may not work with a 10µm MCP-PMT. 12/4/05 J.Va'vra, Japan

40 Silicon PhotoMultiplier (SiPM) (R. Mirzoyan, Max-Planck Inst., IEEE 2005) 42 m SiPMT 1 mm 20 m 1 mm Each pixel = binary device 24*24=576 pixels SiPM = analogue detector Pixels of the SiPM Need a rectangular pixel of 2mm x 8 mm for the Focusing DIRC. Active area presently % Dark count rate/mm 2 : counts/sec at room temperature Single pixel recovery time ~1 sec. A high breakdown probability limits the photon efficiency to ~30% only. 12/4/05 J.Va'vra, Japan

41 Gaseous Micropattern detectors Can they play a role among the fast detectors? Yes, if one can demonstrate three things: (a) longevity of the Bialkali photocathode in the gas, (b) high gain operation, and (c) good timing resolution.

42 Modular setup to test various detector ideas Modular ring structure: Quadruple-GEM + pads: Geometries tested: - Quadruple-GEM + pads - Tripple MCP + pads - GEM + Micromegas + pads - MCP + Micromegas + pads MCP + Micromegas + pads: 12/4/05 J.Va'vra, Japan

43 Micromegas + MCP with a pad readout J.Va vra & T. Sumiyoshi, Nucl.Instr.&Meth. A, 435(2004)334. E Drift-1 E Drift-2 An example of running conditions: E Drift-1 ~350V/cm E MCP ~10kV/cm E Drift-2 ~1.25kV/cm E Micromegas ~50kV/cm Ave. total gain ~2x10 5 Gain distribution in final application: G Micromegas ~2x10 3, G MCP ~100 V Micromegas ~500V, dv MCP ~1200V Photocathode: Metal mesh + Xenon UV light Works well in the single electron mode 12/4/05 J.Va'vra, Japan

44 Mesh and MCP can be made very clean s.s. electro-mesh: MCP: 1000 lpi mesh density (lines per inch) A square hole dimension: ~17 x 17 µm 2 A sidewall width: ~9 µm Made by: BuckBee-Mears Co. A hole diameter: ~50 µm A sidewall width: ~12 µm Thickness: ~1mm Made by: Hamamatsu 12/4/05 J.Va'vra, Japan

45 A good single photoelectron response Vary Micromegas gain mainly: Vary only the MCP gain: Very stable operation even at very high gain in 89.1%He % ic4h10 gas. Observe a slight turnover in the pulse height spectrum. For comparison: Giomataris has observed a clear turnover with ~30% of ic 4 H 10 in the Micromegas alone: 70%He+30%iC 4 H 10 12/4/05 J.Va'vra, Japan

46 MCP with inclined holes + Micromegas J.Va vra & T. Sumiyoshi, NIM A, 435(2004)334 & RICH2004 MCP: 1 dia, 1mm thick, 50micron holes photon MCP with straight holes, B=0kG: MCP with inclined holes, B=15kG: I-Cathode, or I-anode [na] di-cathode di-anode B = 0 kg, V Micromegas = 400 V IBF ~10% Voltage across MCP [V] IBF ~0% IDEA: Block the ion backflow (IBF) by inclined MCP holes in a magnetic field IBF reduction by aligning the MCP holes with the electron s Lorenz angle. Electrons drift & amplify along the MCP hole; ions are caught on the MCP walls. The measured IBF with inclined holes is negligible (consistent with a pa noise). The measured IBF with MCP with the straight holes at a level of ~10%!! No data on electron collection eff. No charging effects observed, which would indicate that the electric field would align with the MCP hole direction. If that would happen, the idea would not work. 12/4/05 J.Va'vra, Japan

47 Inclined MCP holes In this test, use Hamamatsu MCP with a 50µm hole diameter and an angle of 6.5 o. The picture shows a cut through the MCP to verify the angle. The inclined holes are a standard MCP technology as all vacuum-bases MCP- PMTs use them to limit the ion damage of the photocathode 12/4/05 J.Va'vra, Japan

48 Lorenz angle calculation at 15 kg 15kG The MCP angle is fixed by a choice of gas and MCP gain. Use Magboltz program (version 7.1). 12/4/05 J.Va'vra, Japan

49 I-Cathode, or I-anode [na] Experimental setup in the magnet 15kG 90%Ar+10%CH 4 gas Mercury UV lamp MCP needs to be rotated to the optimum azimuth. Indeed, one measures nearly zero cathode backflow current, i.e., consistent with B = 15kG, MCP with 6.5 o hole angles, I-cathode 90%Ar+10%CH - Emcp = 11kV/cm 4 I-cathode - Emcp = 9kV/cm I-anode [na] - Emcp = 9kV/cm I-anode [na] - Emcp = 11kV/cm Arbitrary azimuthal angle [Degrees] a picoammeter noise) at the azimuth angle where the electron transfer is at maximum (aligned with the electron Lorenz angle). 12/4/05 J.Va'vra, Japan

50 Hamamatsu Bialkali GPM R&D work Sumiyoshi, Va vra, Tokanai & Hamamatsu Hamamatsu built a double-mesh Micromegas structure w bialkali pc. Works both in 90%Ar+10%CH 4 or 90%Ar+10%CF 4. No detorioration of the photocathode observed within 5 days Gain of ~6x10 3, limited by secondary effects. Not sufficient for singlephoton detection. Work with MCP & Bialkali photocathode is in progress. 12/4/05 J.Va'vra, Japan

51 Discussion of the gaseous detectors The micro-pattern gas detectors have good aging rate, and can handle high rates (ions travel a short distance). Gaseous detectors could work easily up to 60 kg. Vacuum MCP-PMT will not work much above B~15kG at present. Gaseous detectors can use rather large MCP hole diameter of ~50µm. One could presumably make a large size photon detector using a mosaic of MCPs. Higher geometrical efficiency compared to the vacuum-based MCP- PMTs, at least in principle. Vacuum MCP-PMT has ~50% geometrical efficiency at best. Timing: Giamataris has achieved ~300ps with a Micromegas covered with CsI with just a leading edge disriminator. Adding a MCP will make it worse. The question how much. Needs to be measured. We have invented a simple method to block the ion flow to the cathode. Needs to be studies in more detail using a good simulation code. 12/4/05 J.Va'vra, Japan

52 Conclusion A single photon timing resolution at a level of ~ ps is much closer to a reality compared to a situation when we started. But, much more has to be done. 12/4/05 J.Va'vra, Japan

53 Backup slides

54 Lorenz angle calculation at 15 kg Lorentz angle [Degrees] Lorentz angle [Degrees] B = 15kG, E vs. B angle: 90 o Electric field in MCP hole [kv/cm] B = 15kG, E MCP = 9kV/cm, E vs. B angle: 90 o Methane in Ar/CH4 mix [%] The MCP angle is fixed by a choice of gas and MCP gain. With 90%Ar+10%CH 4 gas & E = 9kV/cm & B = 15kG: v along_e = µm/ns v along_b = 4.21 µm/ns long_along_e ~ 106 µm 2 /ns transv_along_b ~ 245 µm 2 /ns Very high diffusion => expect losses along the MCP hole walls Use Magboltz program (version 7.1). Thanks to Steve Biagi for always making sure that (a) I do it right, and (b) use the latest version of the program. 12/4/05 J.Va'vra, Japan

55 Ion backflow at optimum azimuth is negligible 90%Ar + 10% CH 4 gas The magnitude of the ion backflow at optimum azimuth is zero, consistent with a picoammeter noise. 12/4/05 J.Va'vra, Japan

56 Hamamatsu work with the gaseous photodetectors with Bialkali photocathode So far, they built successfully a Double-mesh Micromegas. Works both in 90%Ar+10%CH 4 or 90%Ar+10%CF 4 gases. Gain of ~6x10 3 reached with a coarser mesh. Coarser mesh yields higher gain (Gain ~ 6x10 3 for 34µm pitch, and Gain ~2x10 3 for 25µm pitch). Not yet good enough for the single electron operation with a good timing resolution. The Micromegas+MCP with inclined holes will be done next. 12/4/05 J.Va'vra, Japan

57 Results with Double Micromegas and Bialkali photocathode in 90%Ar+10%CH 4 gas Cs 137 source, NaI(Tl) convertor, Double Micromegas, P-10 gas QE 90%Ar+10%CF 4 (works as P-10 gas) Serial No. ZX 978 Connect the NaI(Tl) crystal to the Double-mesh Micromegas photodetector operating in the P-10 gas, and with a Cs 137 source obtain the result shown above. QE of Bialkali photocathode in 90%Ar+10%CF 4 gas (the P-10 gas gives similar results): a) 20.8% in vacuum, b) 13% in the gas, c) 20.0% in vacuum again. Wavelength 12/4/05 J.Va'vra, Japan

58 Total gas gain in 94.5%He+5.5%CH 4 gas at 1 bar A factor of ~15 of gain increase every 100 Volts across either the Capillary or the Micromegas in this gas. Use a Mercury UV lamp to do this measurement. 12/4/05 J.Va'vra, Japan

59 Comments on the timing resolution Measurement to produce single electrons off the s.s. mesh using a PiLas laser diode (430nm) was not successful. So, I do not have a direct result, unfortunately. However, perhaps, one could argue theoretically as follows: - Let s assume that the MCP has an average gain of I will use this simple formula: t ~(1/ N) coll /v drift where N = 50, coll =1/ is mean free path ( is Townsend coeff.) and v drift is electron drift velocity in the Micromegas at ~50kV/cm. - Using the Magboltz-Monte program, one obtains t < 100ps for a 90%He+10%CH 4 gas. - However, in addition, there are avalanche fluctuations, which will make it worse. The MCP will also add tails to the timing distribution. 12/4/05 J.Va'vra, Japan

60 How quickly is the ion removed from the insulator? Current [pa] ( ~50 sec) Single-Capillary+Micromegas Quadruple GEM Expon. (Quadruple GEM) Expon. (Single-Capillary+Micromegas) ( ~85 sec) Time [sec] The remnant charge is removed from the insulators of the detector (Kapton or Glass) with a time constant ~85sec for Quadruple-GEM, vs. ~50sec for the Single Capillary + Micromegas detector. Use a Mercury UV lamp (detector draws ~ 350nA). At that point switch lamp off and measure a discharge time constant of the decaying photocurrent. 12/4/05 J.Va'vra, Japan